To explain a typical example of known light triggered thyristors, reference will be made to FIGS. 1 (a) and (b), which schematically show a vertical cross-section of its light-receiving section and a plan view thereof, respectively. An n-type semiconductor substrate 1 has a p-type, that is, first electrically conductive type base region 3 in its main surface 2, and a p-type emitter region 4 in the opposite surface 4. The p-type base region 3 has a light-receiving surface 5 having a radius of (a), around which an n-type, that is, second electrically conductive type emitter region 6a of a ring shape is concentrically provided. The ring-shaped emitter region 6a has an inside radius of (a), and an outside radius of (c). The reference numeral 7 designates a negative electrode of a ring shape having an outside radius of (b) and an inside radius of (d), and negative electrode 7 being kept in surfacial contact with the n-type emitter region 6a and with the p-type base region 3 outside the n-type emitter region 6a so as to secure electrical connection to each of them. The reference numeral 8 designates a positive electrode provided on the surface of the p-type emitter region 4. In addition, there is provided a light guide 9 above the light-receiving surface 5. The light passing through the light guide 9 is cast on the light-receiving surface 5 as a trigger light 10.
When the light triggered thyristor constructed in the above-mentioned manner is biased, and the trigger light 10 is projected on the light-receiving surface 5 through and from the light guide 9, the trigger light 10 penetrates the light-receiving surface 5, and generates a light-excited current 11a inside the p-type base region 3. This current flows therethrough, and reaches the negative electrode 7. At this stage, a voltage drop occurs in that area of the p-type base region 3 which is located below the n-type emitter region 6a, because of a lateral resistance R.sub.A existing in this area, thereby causing the junction between the p-type base region 3 and the n-type emitter region 6a to be forward-biased. Thus electrons are injected into the p-type base region 3 from the n-type emitter region 6a, whereby the light triggered thyristor is switched on. At this moment the minimum light-triggering energy P.sub.LT is expressed by the equation (1): ##EQU1## Where V* : the critical voltage for causing the injection of electrons;
y.sub.A : quantum yield; PA1 r.sub.S : a sheet resistance in the p-type base region under the n-type emitter region PA1 a light-receiving surface for receiving incident; PA1 a base region of a first electrically conductive type provided in a main surface of a semiconductor substrate such that the base region is exposed at the light-receiving surface; PA1 an emitter region of a second electrically conductive type which is exposed at the central portion of the light-receiving surface as well as in an area extending from a part of the central portion thereof outwardly beyond the light-receiving surface; PA1 an electrode connected to the base region and the emitter region outside the light-receiving surface, wherein a lateral resistance in the area of the base region between the emitter regions and extending from the light-receiving surface up to the electrode is approximately equal to or larger than a lateral resistance in the area thereof below the emitter region.
In order to increase the sensitivity of the light-triggering; in other words, to reduce the value of the P.sub.LT, it is necessary to increase the R.sub.A. FIG. 2 shows a modified light-receiving section in the known light triggered thyristor, in which the n-type emitter region 6b is expanded so as to cover the area under the light-receiving surface 5. The remaining structure is the same as that illustrated in FIG. 1. By virtue of this arrangement a lateral resistance R.sub.B in the p-type base region 3 under the n-type emitter region 6b becomes larger than the R.sub.A referred to with respect to FIG. 1. Here, the minimum light-triggering energy P.sub.LT is given by the equation (2): ##EQU2## where y.sub.B : quantum yield
Now, suppose that the values of r.sub.S and V* are the same as those shown with respect to FIG. 1, and that y.sub.A =y.sub.B is established, it will be appreciated from the comparison of both equations (1) and (2) that the structure of the light-receiving section illustrated in FIG. 2 is superior in light-triggering sensitivity to that of FIG. 1. In fact, however, y.sub.A is larger than y.sub.B. The reason is that under the structure illustrated in FIG. 1 the p-type base region 3 is exposed in the whole light-receiving surface 5, whereby the trigger light 10 is wholly used to generate the light-excited current 11a. Whereas, under the structure illustrated in FIG. 2 the whole light-receiving surface 5 is covered with the n-type emitter region 6b, whereby the trigger light 10 must penetrate the n-type emitter region 6b so as to reach the p-type base region 3. During the penetration the trigger light 10 is partially absorbed in the n-type emitter region 6b, and the absorbed portion hardly contributes to the generation of the light-excited current 11b. This prevents the enhancement of the light-triggering sensitivity, which is quite contrary to expectation.
As evident from the foregoing description, the known light triggered thyristors have a low light-triggering sensitivity; in the example illustrated in FIG. 1 the lateral resistance in the area of p-type base region 3 below the n-type emitter region 6a is too small to increase the sensitivity. In the modified example of FIG. 2 the quantum yield is reduced, which results in a low sensitivity. To increase the quantum yield, it is recommended to intensify the trigger light 10, which, however, leads to a costly optical system.